MAGNETOHYDRODYNAMIC WIND-DRIVEN BY COSMIC-RAYS IN A ROTATING GALAXY

We have calculated the dynamics of mass loss from a disk galaxy with t he escaping high energy component of the ISM (cosmic rays), and the ma gnetic field. The coupling between cosmic rays and plasma is provided by the resonant excitation of small-scale magnetic field fluctuations (MHD waves). Earlier work is generalized by including galactic rotatio n and the resulting magnetic tension of the frozen-in field. It is sho wn that nonlinear damping of self-excited waves in the galactic halo i s significant, thus heating the plasma. We obtain steady-state galacti c wind solutions with a moderate enhancement of the galactic mass loss rate in comparison with a non-rotating and undamped model. A qualitat ive new result of this theory is the determination of the angular mome ntum loss rate of a galaxy. Due to magnetic stresses, this exceeds the angular momentum carried away by the fluid elements leaving the galax y. Thus the halo is forced to corotate with the disk up to a vertical distance of a few kiloparsecs, typically. Quantitatively, for our Gala xy, we find that over a Galactic evolution time scale of 10(10) years about 17 to 39 percent of the total angular momentum (including the st ellar component) is lost. The corresponding numbers due to magnetic st resses alone are about 2 to 17 percent. Since the star formation rate is likely to have been significantly larger in the past, it seems inev itable that galaxies like ours have lost a significant amount of their present angular momentum.